1. Field of the invention
The present invention relates to a novel charge transporting copolymer applicable to electronic devices in which the charge transporting function is utilized and which are exemplified by an electrophotographic photoreceptor, an electroluminescence element, a photorefractive element, an optical sensor and a photocell, a method for making the novel charge transporting copolymer, and an electrophotographic photoreceptor and an electrophotographic apparatus utilizing the novel charge transporting copolymer.
2. Description of the related art
Recently, because of the advantages of high speed and high-quality print, electrophotography is playing a major role in such applications as copying machines, printers and facsimiles.
In the electrophotographic technologies, well-known photoreceptors are those which utilize inorganic photoconductive materials such as selenium, selenium-tellurium alloys, and selenium-arsenic alloys.
Meanwhile, electrophotographic photoreceptors utilizing organic photoconductive materials, which are far more advantageous in terms of cost, production and waste disposal than the above-mentioned inorganic photoconductive materials, are now being actively studied. And at present, organic photoreceptors such as these have become superior to inorganic photoreceptors. Particularly, owing to the development of a function separated multilayer photoreceptor in which two separate functions that constitute the basic process as for photoconductance, i.e., the generation of photoelectric charge and the transport of the charge, are performed by separate layers, the latitude for the selection of materials has been widened and a remarkable improvement of the performance has been achieved. This is because, designing and developing a material excellent both in the charge generation and the charge transport had been problematic, however, in the case of the function separated multilayer photoreceptor, a high performance electrophotographic photoreceptor was able to be obtained thanks to the development of materials excellent in any one of the functions. Accordingly, the main stream electrophotographic photoreceptor is currently this function separated multilayer photoreceptor.
Practical examples of the charge generating layer for use in the function separated multilayer organic photoreceptor consist of a layer produced by a direct vapor deposition of a pigment having excellent charge generation functions such as quinone pigments, perylene pigments, azo pigments, phthalocyanine pigments and selenium pigments, or a layer made up of any of these pigments dispersed at a high concentration in a binder resin.
However, as the material which forms the charge transporting layer, originally, use was made of a charge transporting polymer such as polyvinyl carbazole (PVK) which combined a charge transporting function with a film forming capability. However, PVK is associated with inherent problems such as the mechanical strength being insufficient and the mobility of the charge being slight. In order to solve these problems, function separated charge transporting layers to support the functions required of the charge transporting materials, i.e., a charge transporting function and mechanical properties, such as film forming capability, flexibility and strength, as well as the materials thereof, were developed. As a result, the main stream charge transporting material for use in an electrophotographic photoreceptor is currently a composite material made by dispersing a low-molecular-weight compound such as a hydrazone compound, a benzidine compound, an amine compound and a stilbene compound, which are each excellent in charge transporting functions, in an insulating resin having excellent mechanical properties.
However, the above-mentioned composite material made up of a dispersion of a low-molecular-weight compound presented the problem that the dispersed charge transporting low-molecular-weight compound crystallized with a lapse of time and/or under heat and thus the properties deteriorated. Especially, the material could not be used at a high temperature or in a device such as an electroluminescent element which generates heat (Joulean heat). A second problem was that, if the material was used in a photoreceptor in an electrophotographic apparatus utilizing a developing liquid to obtain a high-quality image, the contact of the material with the developing liquid caused cracks to form in the material or its properties to change due to the dissolution or crystallization of the charge transporting low-molecular-weight compound. A third problem was that, since it was necessary to disperse the low-molecular-weight compound performing the charge transporting in a resin at a concentration as high as 35% to 60% by weight for securing a sufficient charge transporting function, it was difficult to obtain a composite film excellent in mechanical properties, even if the resin had excellent mechanical properties such as flexibility and strength.
The above-mentioned problems need to be overcome, for an organic charge transporting material to be used widely in such applications as photoreceptors for electrophotography and organo-electronic devices where a higher performance, a longer service life, a higher durability, a higher speed and a lower cost are required.
Meanwhile, in response to the recent demand for high-quality images, high added value and network, digital system electrophotographic apparatuses are being actively studied and developed. In a conventional analog system electrophotographic copying machine, in which a photoreceptor is exposed by optically imaging an original on the photoreceptor, a photoreceptor is required which has a photo-induced potential attenuation characteristic as shown in FIG. 1, namely, a photoreceptor whose potential attenuates proportionally to the exposure amount (hereinafter referred to as "J-type photoreceptor"), in order to improve the reproducibility of halftone by density gradation. The aforementioned inorganic photoreceptors and organic function separated multilayer photoreceptors exhibit photo-induced potential attenuation characteristics as shown in FIG. 1. However, in a digital-system electrophotographic apparatus generally utilizing an area gradation method in which the gradation is produced by the area percentage of dots or the like, in order to increase the sharpness of pixels, it is desirable to use a photoreceptor which has a photo-induced potential attenuation characteristic as shown in FIG. 2, i.e., a photoreceptor exhibiting no attenuation of potential until a certain exposure amount is reached, but once that exposure amount is reached then the attenuation of potential is abrupt (hereinafter referred to as "S-type photoreceptor").
The S-type photo-induced potential attenuation characteristic is a known phenomenon in a single-layer photoreceptor made by dispersing the particles of an inorganic pigment such as ZnO or an organic pigment such as phthalocyanine in a resin {For example, R. M. Schaffert, "Electrophotography", Focal Press, pp.344(1975), and J. W. Weigl et al., "Current Problems in Electrophotography", Walter de Gruyter, pp.287(1972)}. In particular, many of the single-layer photoreceptors designed for exposure to laser are proposed, and these photoreceptors are produced by dispersing a phthalocyanine pigment, which has a sensitivity in the near-infrared region which is the wavelength region of a semiconductor laser now in wide use in many applications, in a binder resin {For example, G. C. Que et al., "J. of Chem. Soc. of Japan, pp.393(1986), Japanese Patent Application Laid-Open (JP-A) Nos. 1-169,454, 2-207,258, 3-31,847 and 5-313,387}. These single-layer photoreceptors are required to perform the two functions, i.e., charge-generation and charge transport, in a single material.
However, a material which is excellent in the two functions is rare, and no material able to endure actual use has been obtained yet. Generally, pigment particles have many trap levels, and therefore are not suitable for use as a charge transporting material because of the drawbacks such as poor performance in charge transporting, residual charge and poor stability in repeated use. In order to fundamentally solve these problems, broaden the latitude for the selection of materials, and enhance the overall properties of the photoreceptor, it is essential that a function separated structure be introduced in an S-type photoreceptor as well.
In this connection, D. M. Pai et al. report that, if a heterogeneous charge transporting layer, which comprises at least two charge transporting regions and one electrically inert region so that the charge transporting regions contact each other to form a helical charge transporting pathway, is combined with a charge generating layer to form multilayer photoreceptor having two layers composed of a charge generating layer and a charge transporting layer, it is possible to produce S-type photo-induced potential attenuation characteristic {JP-A No. 6-83,077 (U.S. Pat. No. 5,306,586). Meanwhile, the present inventors have succeeded in increasing the degree of separation of functions more than that of the above-mentioned invention of D. M. Pai et al., based on the discovery that a three-layer photoreceptor, which comprises a charge generating layer, a heterogeneous charge transporting layer and a homogeneous charge transporting layer, exhibits S-type photo-induced potential attenuation characteristics (JP-A No. 9-96,914).
However, the disclosed examples of the heterogeneous charge transporting layer which produces the S-type photo-induced potential attenuation characteristics, which is the key to the invention, (hereinafter referred to as "S-shaping") are limited to a layer composed of a resin having fine crystals of a phthalocyanine pigment dispersed therein, a layer composed of a resin having fine hexagonal crystals of selenium dispersed therein, and a phase-separation block copolymer of polyvinyl carbazole/dodecyl methacrylate. And, since these materials have the problems as described below, it was essential to develop a novel material for the heterogeneous charge transporting layer in order to put the above-mentioned invention to a practical use.
Namely, if a coloring pigment, such as a phthalocyanine pigment, hexagonal selenium or the like, which has a charge generating function, is used as a charge transporting material in the heterogeneous charge transporting layer, and if the charge generating layer is exposed to light particularly from the side of the charge transporting layer, the absorption of light in the heterogeneous layer and the resultant charge generation causes the problems of the sensitivity to light and chargeability being adversely affected and the stability in repeated use decreases. Although these problems maybe avoided by use of a charge generating layer, which has a sensitivity to light in the wavelength region not absorbed in the heterogeneous charge transporting layer, and by use of an exposing device, which performs the exposure only within the foregoing wavelength region, a constraint is imposed on the material and the device. In addition, when producing a layer composed of a resin having fine crystals of phthalocyanine pigment dispersed therein, or a layer composed of a resin having fine hexagonal crystals of selenium dispersed therein, it is necessary that these pigments be ground and/or dispersed, however, the grinding operation needs an enormous amount of energy and involves the risk of the blending in of impurities. The selection of suitable solvents and binder resins is a laborious task and the thus selected materials are significantly limited, because it is generally difficult to obtain a stable dispersion liquid. Further, since the denaturation of the dispersion liquid due to the growth of crystals and flocculation is generally unavoidable, the dispersion cannot be used for a long period of time. Accordingly, frequent replacement of the dispersion liquid is necessary, leading to cost increases.
Meanwhile, the advantages of the phase-separated block copolymer of polyvinyl carbazole/dodecyl methacylate are that the above-mentioned problem related to the grinding and/or dispersing is fundamentally solved, because the coating solution is homogeneous and the drying of the solution gives rise to the micro-separation of phase that produces the desired heterogeneous charge transporting layer. Further, the problem related to the above-mentioned absorption of light does not occur, because the phase-separation block copolymer is not a pigment. However, a problem of the phase-separation block copolymer is that the production is difficult and expensive, because the copolymer is produced by use of a special bifunctional initiator. Other problems of the copolymer are low mechanical strength, low mobility of charge and insufficient charge injecting capability.
As stated above, in recent years, in response to demand for organo-electronic devices and for digital system electrophotographic apparatuses, a high-performance charge transporting polymer to replace polyvinyl carbazole is once more being actively studied. Since a triarylamine-based charge transporting compound having a low molecular weight has a high charge transporting capability, many charge transporting polymers having a triarylamine skeleton in a main chain or a side chain have been developed as at the present time.
For example, U.S. Pat. No. 4,806,443 discloses charge transporting polycarbonate obtained by the polymerization of a specific dihydroxytriarylamine with bischloroformate, while U.S. Pat. No. 4,806,444 discloses a charge transporting polycarbonate obtained by the polymerization of a specific dihydroxyarylamine with phosgene. U.S. Pat. No. 4,801,517 discloses a charge transporting polycarbonate obtained by the polymerization of a bis(hydroxyalkyl)triarylamine with bischloroformate orphosgene, while U.S. Pat. Nos. 4,937,165 and 4,959,288 disclose a charge transporting polycarbonate obtained by the polymerization of a specific dihydroxyarylamine or bis(hydroxyalkyl)triarylamine with bischloroformate, and a charge transporting polyester obtained by the polymerization of a specific dihydroxyarylamine or bis(hydroxyalkyl)triaryl amine with a bisacyl halide. U.S. Pat. No. 5,034,296 discloses a charge transporting polycarbonate and a polyester containing a triarylamine structure having a specific fluorene skeleton. U.S. Pat. No. 4,983,482 discloses a charge transporting polyurethane. JP-A Nos. 61-20,953, 1-134,456, 1,134,457, 1-134,462, 1-133,065 and 4-133,066 disclose charge transporting polymers having a charge transporting skeleton such as hydrazone and triarylamine in a side chain, and an electrophotographic photoreceptor utilizing such charge transporting polymers.
However, it is still difficult to realize all of the required properties with a homopolymer composed of a single constituent component. Consequently, a satisfactory material has not yet been obtained.
In order to solve the above-mentioned problems, copolymers composed of several monomers are proposed by, for example, JP-A Nos. 4-11,627, 7-72,640, 6-256,428, 5-,295,096, 5-310,904, 5-331,238 and 5-202,135. These copolymers, however, are produced by a single-stage polymerization wherein raw material monomers are used in the form of a mixture, and therefore the copolymers are random copolymers in which the monomers are randomly linked. Because of this, these copolymers cannot fully exhibit the properties where each of the monomers is singly polymerized, and accordingly the above-mentioned problems still remain unsolved. That is, if a homopolymer composed of a charge transporting monomer is inferior in flexibility, the flexibility can be improved by a random copolymer composed of the same charge transporting monomer and a flexible monomer. However, the copolymer thus obtained will have the concentration of the monomer active in the charge transporting reduced by the added monomer, and the function of charge transporting will decrease proportionally. Further, if the component of the copolymer forms a charge trap, a new problem will arise which impairs the charge transporting performance more than the above-mentioned dilution of the charge transporting performance.
Moreover, since these random copolymers do not exhibit any substantive phase separation, singly they are unable to form a heterogeneous charge transporting layer for S-shaping.
As for the S-shaping, the present inventors found that a phase-separated polymer blend, which was prepared from the charge transporting polymer and an insulating polymer incompatible with the charge transporting polymer, functioned effectively as a heterogeneous charge transporting layer for S-shaping (Japanese Patent Application Nos. 8-58,858 and 8-158,520).
However, since the above-mentioned polymer blend generally caused the phase separation in a scale of several .mu.m or more, the homogeneity of the image decreased. Further, manufacturing reproducibility problems also existed, namely that, depending on the film forming conditions such as drying speed and drying temperature, the scale of phase separation varied significantly and the electrophotographic properties dependent thereon also varied significantly.